1. Field of the Invention
The present invention relates to a heat pipe circuit board having its own cooling function for improving the performance of dissipating heat generated by electronic components.
2. Related Art
In these days, as represented by developed personal computers, CPU, electronic devices, other electronic components (hereinafter, these are all referred to as xe2x80x9celectronic componentsxe2x80x9d) are downsized with significant improvement in performance. As electronic components become more sophisticated, an amount of heat generated by the electronic components is more increased. Therefore, it is now one of most serious technical problems how to efficiently dissipate heat generated by electronic components, to prevent increase in temperature of the electronic components and to keep high performance of the electronic components.
An electronic component is usually mounted on a printed board to constitute a predetermined electronic circuit with other electronic components. The printed board is a thin plate made of an insulating material. On the surface of the printed board, there is printed a metal material with high electrical conductivity, such as copper or the like, so as to constitute a predetermined circuit. The printed board has a hole at a position where an electronic component is mounted, and the electronic component is jointed onto the printed board by soldering or another jointing method to make up a predetermined electronic circuit. However, the printed board itself has low heat conductivity and does not have any particular cooling function.
When an amount of heat generated by electronic components is not large, the heat can be dissipated by natural convection without any particular cooling device being provided. When the heat amount exceeds some degree, it becomes required to actively contain additional heat-dissipating means. Conventionally, as typical heat-dissipating means, there is a method for connecting a heat sink which is made of metal having high heat conductivity. Further, this method is often used in combination with cooling by a cooling fan.
However, recently, as electronic components are more downsized and sophisticated, the demand is growing for a space-saving heat dissipating method having high heat-dissipating efficiency. As a typical cooling method or heat dissipating method, there is a method of using a heat pipe.
When an electronic component is large to some degree, a heat pipe is provided in direct contact with the electronic component. For a small-sized electronic component, a heat pipe is often provided in contact with a printed board on which the electronic component is mounted.
As a typical example of the conventional art, description below is made about a method for cooling a circuit board which is a printed board with electronic components mounted on by bringing a heat pipe in contact with the circuit board.
The heat pipe is typically made of metal having relatively high heat conductivity such as copper or aluminum. Between the printed board and the heat pipe, an insulating material having extremely high heat conductivity is inserted to achieve excellent heat conductivity while keeping electrical insulation.
In addition, the printed board and the heat pipe are jointed by a mechanical joint method, a method of using an adhesive agent or the like. In the case of the method of using an adhesive agent, the adhesive agent also performs the function of the above-mentioned insulating material having high heat conductivity.
Heat generated by electronic components passes through the printed board and the insulating material inserted between the printed board and the heat pipe, then to be transferred to an outer wall of a heat receiving portion of the connected heat pipe. The heat is further transferred from the outer wall of the heat receiving portion of the heat pipe to an inner wall thereof then to be conducted to a working fluid stored in the heat receiving portion in a fluid channel of the heat pipe. The working fluid is vaporized by the conducted heat, and then, the vaporized working fluid is moved to reach a heat dissipating portion of the heat pipe. An outer wall of the heat dissipating portion is provided with heat dissipating fins or the like so as to dissipate heat to the outside.
The vaporized working fluid is deprived of heat by being brought into contact with an inner wall of the heat dissipating portion of the heat pipe, and the heat deprived of from the working fluid is transferred from the inner wall of the heat pipe to the outer wall to be dissipated into the air by external cooling fins or the like. In order to improve heat dissipating efficiency, a cooling fan is sometimes arranged.
The working fluid deprived of heat at the heat dissipating portion is changed from gas back to liquid. The liquefied working fluid flows in the fluid channel back to the heat receiving portion by gravity or capillary force. Then, the above-described cycle is repeated.
Since a heat pipe utilizes evaporation heat of a working fluid, it is a method which enables efficient heat transfer. However, heat generated by electronic components is transferred to a heat pipe via a printed board. Since the heat conductivity of the printed board per se is not as high as that of metal and the like, much thermal resistance exists between the electronic components and the heat pipe. Therefore, even if a high-performance heat pipe is installed, this thermal resistance may prevent heat generated by the electronic component from being dissipated efficiently.
As a typical manufacturing method of a flat-plate-type heat pipe, there is a method of preparing two flat metal plates of which at least one has a recess portion, which is a fluid channel of a working fluid, formed by machine working, bending or the like, jointing the two flat metal plates and injecting a working fluid into the recess portion.
Jointing methods of metal plates include welding, brazing, pressure-bonding and the like. For the case of welding, since raw materials are fused to be jointed, a large amount of heat is given to the metal plates, which results in creating distortion due to the heat in the metal plates. Besides, there occur bumps in a welded portion of the metal plates, which can not be smooth.
Accordingly, it is difficult to have a smooth contact surface between the printed board and the heat pipe, thereby preventing heat from being transferred from the printed board to the heat pipe. In addition, since the metal plates are subjected to high heat in being welded, it is impossible to mount an insulating material on the metal plate in advance or to joint the metal plates after the metal plates are mounted on the printed board.
Further, since in the welding method, there occurs sputtering gas or welding gas and much energy is applied, additional cost, time and effort may be necessitated to create an appropriate clean environment that handles electronic components. Furthermore, the welding method needs additional materials including a welding rod, shielding gas and the like.
In the case of brazing, there is no need to fuse raw materials. However, the brazing method is a high-temperature process at a temperature of 600 degree or higher, which is not higher than the temperature of welding method, but is high enough to produce distortion in the metal plates. Besides, there occur bumps in a brazed portion of the metal plates, which can not be smooth. Therefore, there arise substantially the same problems as those in the welding method.
In the case of pressure-bonding, there does not occur distortion due to exposure to high heat nor bumps due to welding or the like as mentioned above. However, the joint strength in pressure-bonding is lower than that of the other methods, there may arise a problem of leakage of a working fluid in the metal plates. Therefore, this method is not suitable for use in a situation of exposure to thumps, bumps and vibration.
As described above, there are the following problems in the conventional method of cooling a circuit board by providing a heat pipe in contact with the circuit board.
Because of a large thermal resistance of a printed board made of an insulating material, heat generated by electronic components is prevented from being transferred to a heat pipe connected to the printed board. Therefore, it may become impossible to adequately cool ever-increasing heat generated by electronic components.
In addition, since there occurs distortion, bumps or the like in a heat pipe manufactured by welding or brazing, it is impossible to improve adhesiveness between a circuit board and the heat pipe, and therefore, heat generated by electronic components is prevented from being transferred to the heat pipe efficiently.
Further, according to the above-mentioned welding and brazing methods, since plate members comprising a heat pipe are exposed to high temperature, it is impossible to joint the plate members after they are mounted onto a circuit board and the like.
Furthermore, according to the above-mentioned welding and brazing methods, there occurs sputtering gas or welding gas and much energy is applied, which is suitable for the clean environment that handles electronic components. Therefore, additional time, effort and cost may be required to create an appropriate environment.
Still furthermore, a heat pipe manufactured by pressure-bonding does not have such problems as distortion, bumps, high heat in welding and an environmental problem that are shown in the welding and brazing method. However, the joint strength in pressure-bonding is lower than that of the other methods, and therefore, the pressure-bonding method is not suitable for use in a situation of exposure to thumps, bumps and vibration.
Therefore, it is an object of the present invention to solve the above-mentioned problems to provide a cooling device equipped circuit board which is capable of efficiently cooling heat generated by electronic components and of resisting thumps, bumps and vibration.
The inventors of the present invention have intensively studied in order to solve the above-mentioned problems. As a result, they have found that a heat pipe circuit board described below is capable of transferring heat generated by electronic components to a cooling device more efficiently than the conventional art to enhance cooling performance. In addition, by adopting fiction stir welding which does not generate high heat in jointing, they have achieved a heat pipe circuit board manufacturing method of jointing the heat pipe plates after electronic components are mounted thereon.
A first embodiment of the heat pipe circuit board according to the present invention is a heat pipe circuit board comprising: a heat pipe; at least one insulating layer; a circuit pattern provided on a surface of or inside said at least one insulating layer; and electronic components being mounted on said heat pipe on said insulating layer.
A second embodiment of the heat pipe circuit board according to the present invention is a heat pipe circuit board in which said heat pipe is formed by jointing two plate members by friction stir welding, at least one of said two plate members being provided with a recess portion which is a fluid channel of a working fluid.
A third embodiment of the heat pipe circuit board according to the present invention is a heat pipe circuit board in which said at least one insulating layer takes the shape of a sheet or a plate.
A fourth embodiment of the heat pipe circuit board according to the present invention is a heat pipe circuit board in which said at least one insulating layer is formed by a coating.
A fifth embodiment of the heat pipe circuit board according to the present invention is a heat pipe circuit board in which said at least one insulating layer is strong enough to support said electronic components by itself.
A sixth embodiment of the heat pipe circuit board according to the present invention is a heat pipe circuit board in which said heat pipe includes a cooling surface, said cooling surface performing the function of a mounting bracket for mounting said peat pipe on another member.
A first embodiment of the heat pipe circuit board manufacturing method according to the present invention is a heat pipe circuit board manufacturing method wherein two plate members are prepared, at least one of the plate members being provided with a recess portion which is a fluid channel of a working fluid, electronic components being mounted on at least one of the plate members via an insulating layer, and then, the plate members being jointed by friction stir welding.
Another embodiment of the heat pipe circuit board according to the present invention is a heat pipe circuit board in which said electronic components are mounted on said two plate members via said at least one insulating member.
Still another embodiment of the heat pipe circuit board according to the present invention is a heat pipe circuit board in which said electronic components are mounted on either of said two plate members via said at least one insulating member.